Environmental Engineering Reference
In-Depth Information
the physicochemical properties of man made surfaces by the functionalization
of these surfaces, yielding formation of 'tailor-made' surfaces. Chemical trans-
formations on SAMs have been studied in detail and provide new mechanistic
insights as well as routes to tailored surface properties [49, 50, 51]. Methodol-
ogies of surface modification of SAMs focus on two strategies: (a) chemical
modification and attachment of organic molecules after formation of SAMs
and (b) attachment prior to assembly, i.e., the desired attachment on alka-
nethiols are carried separately in solution and the synthesized molecules are
then assembled on gold surfaces. This review focuses on chemical modification
of SAMs after assembly to a substrate.
Variation of the head group of the monolayer makes it possible to control
wettability, etc., and also allows the introduction of different chemical moieties
with specific properties such as non-specific binding of proteins to surfaces. For
example, the introduction of oligoethylene glycol functionality to the end of the
alkyl chain results in protein-resistant properties [52]. Thus instead of synthe-
sizing different thiols/silanes with different head groups, it is more convenient
to use a number of 'standard' SAMs and subsequently perform reactions on
SAMs to modify the surface chemistry. Performing reactions on SAMs allows
us to tune the properties of surfaces at the molecular level, but due to the nature
of SAMs (tightly packed, movements of molecules within monolayers
restricted) the choice of reaction is important. One must consider that steric
effects are likely to be exacerbated for certain surface reactions, leading to an
energy barrier higher than would be expected in solution chemistry. To success-
fully functionalize a SAM, reaction conditions must not cause destruction of
the monolayer or damage the underlying substrate.
Over the last decade, a considerable number of reactions have been studied
[49, 50]: (i) olefins: oxidation [53, 54], hydroboration, and halogenation [55];
(ii) amines: silylation [56, 57], amidation [58], and imine formation [59]; (iii)
hydroxyl groups: reaction with anhydrides [60], isocyanates [61], epichlorohy-
drin and chlorosilanes [62]; (iv) carboxylic acids: formation of acid chlorides
[63], mixed anhydrides [64] and activated esters [65]; (v) carboxylic esters:
reduction and hydrolysis [66]; (vi) aldehydes: imine formation [67]; (vii) epox-
ides: reactions with amines [68], glycols [69] and carboxyl-terminated polymers
[70]. A list of all the major classes of reactions on SAMs plus relevant examples
are discussed comprehensively elsewhere [50]. Section 3.2.6 will provide a more
detailed look at reactions with some of the common functional SAMs, i.e.,
hydroxyl- and carboxyl-terminated SAMs.
The surface modification of polymers with self-assembled molecular struc-
tures has also been studied. Ratner and co-workers [71] have described a simple,
one-step procedure for generating ordered, crystalline methylene chains on
polymeric surfaces via urethane linkages. The reaction of dodecyl isocyanate
with surface hydroxyl functional groups, catalyzed by dibutyltin dilaurate,
formed a predominantly all-trans, crystalline structure on a crosslinked poly
(2-hydroxyethyl methacrylate) (pHEMA) substrate was demonstrated. X-ray
photoelectron spectroscopy and time-of-flight SIMS showed that the surface
